1. Technical Field
The present invention relates to an actuator, an optical scanner and an image-forming device.
2. Related Art
JP-A-2004-191953 is a first example of related art, and JP-A-2005-326463 is a second example of the related art.
As an example of an optical scanner which is for drawing an image through optical-beam scanning and equipped in a laser printer or the like, one using an actuator having a torsional vibrating element has been known (see the first example.)
The first example discloses an actuator having a torsional vibrating element of a single-degree-of-freedom vibration system. This actuator has the torsional vibrating element of a single-degree-of-freedom vibration system that includes a weight member, a fixing frame and a pair of torsional springs that couples the weight member rotatable to the fixing frame. The both sides of the weight member are supported by the torsional springs. A light reflecting part that has a light reflectivity is provided on the weight member. The weight member turns when the pair of the torsional springs is distorted and deformed. Light is reflected at the light reflecting part and the scanning is performed. In this way, an image can be drawn through the optical-beam scanning.
In this actuator, the weight member, the pair of the torsional springs and the fixing frame are fabricated together so as to form a single body by etching a silicone substrate (a silicon wafer).
When the weight member should be turned rapidly, a torsional spring constant of the elastic part has to be high. There are two ways, for example, to make the torsional spring constant high: one is to make the thickness of the elastic part larger, and the other is to make the longitudinal length of the elastic part shorter.
However, when the above-two ways are applied to silicon which is relatively hard material, the amount of a change in the torsional spring constant with respect to a change in the length or/and the thickness of the elastic part becomes too large. In other words, it is difficult to perform a fine adjustment of the torsional spring constant at the elastic part.
When the weight member should be turned slowly, the torsional spring constant of the elastic part has to be low. There are two ways, for example, to make the torsional spring constant high: one is to make the longitudinal length of the torsional spring longer, and the other is to increase the weight of the weight member.
However, the above-two ways increases the size of the actuator. This means that it is difficult to drive the weight member slowly while downsizing the actuator at the same time. For these reason, the actuator according to the first example has difficulty in downsizing and the fine adjustment of the torsional spring constant. Therefore, the actuator according to the first example cannot be accommodated for many applications (for example a high-speed driving actuator, a low-speed driving actuator and the like).
The second example discloses an optical deflector having a torsional vibrating element of a single-degree-of-freedom vibration system. The optical deflector has the torsional vibrating element of the single-degree-of-freedom vibration system which includes a movable plate of which two sides are held by elastic support members (torsional springs). A reflective face (a micro-mirror) having a light reflectivity is provided on the movable plate. The movable plate turns when the elastic support members are distorted and deformed. Light is reflected at the reflective face and the scanning is performed. In this way, a image can be drawn through the optical-beam scanning.
In the optical deflector, a strain gauge whose resistance changes corresponding to the degree of the distortion of the elastic support member is provided. A torsional strain detection circuit detects the variation in the resistance of the strain gauge and a driving circuit of the optical deflector is controlled based on the results obtained through the detection circuit. Thereby a highly accurate scanning can be performed.
However, according to the second example, the torsional strain detection circuit or/and the driving circuit are not provided in the optical deflector itself. Therefore, the size of the deflector will become large if these are installed in the deflector and it is difficult to downsize the deflector.